Field of the Invention
The invention relates to an apparatus and method for reading optical information, in particular an apparatus and a reading method with an interface with reduced visual fatigue.
Description of Related Art
Reading apparatuses are known for reading optical information found in an object that comprise: optical information acquisition means; illuminating means associated with this acquisition means for emitting, during use by a user, an illuminating pattern of a zone that is at least equal to the field of view of the acquisition means in which this optical information is positionable and thus enabling the acquisition thereof; aiming means for emitting at least one luminous aiming pattern in a space in front of a frontal face of the apparatus, to guide a user in positioning the optical information inside the field of view of the acquisition means; and indicating means of a reading outcome to emit a reading outcome pattern that is suitable, for example, for providing an indication that is green in colour to indicate a positive reading outcome. The illuminating pattern, the aiming pattern and the reading outcome pattern define an HMI (Human Machine Interface) of the reading apparatus based on the visual perception by the user, to guide a user in reading optical information found in an object correctly.
These reading apparatuses can be, for example, of fixed type, and be, for example, connected to a supporting base the position of which does not vary over during operation of the apparatus or be portable reading apparatuses that are holdable by a user. In both cases, the user has to position the optical information in the object in such a manner that it is framed by the acquisition means of the reading apparatus, by moving the object to arrange the optical information in the field of view of the acquisition means or, in the case of portable reading apparatuses, by moving the portable device to frame the optical information as well as possible.
The expression “optical information” means any graphic representation that constitutes information, whether coded or uncoded.
One example of coded information is an optical, linear or two-dimensional code, in which the data are coded that identify the object with which the optical code is associated. The information is coded by suitable combinations of elements of a preset shape, for example squares, rectangles or hexagons, of a dark colour (normally black) separated by light elements (spaces that are normally white), and barcodes, stacked codes and two-dimensional codes in general, colour codes, etc, are known.
The term “optical information” further comprises, more in general, also other graphic shapes, which include printed or handwritten characters (letters, numbers, etc) and particular shapes (so-called “patterns”), such as, for example, stamps, logos, signatures, fingerprints, etc and any graphic representation that is detectable not only in the field of visible light but also along the entire wavelength comprised between infrared and ultraviolet.
In the case of coded information, whether it be linear or two-dimensional codes or graphic shapes, the acquisition means for acquiring optical information can be made of electronic sensor image acquisition devices having, for example, an array of to photosensitive elements of linear or matrix type for example of the CCD or CMOS type, that are able to acquire images of an object, and in particular to process said images in such as manner as to extract optical information therefrom that is associated with the object.
The reading apparatus further comprises a control device, for commanding the acquisition of the image, emitting the illuminating pattern, emitting the aiming pattern and the reading outcome pattern within the appropriate time and processing and/or decoding the acquired optical information.
“Field of view” is to be understood as an acquisition field of the reading apparatus, i.e. a preset area inside which the images of the object can be acquired, which is located within a focusing range and for which, along the optical axis of the optical receiving means, it is possible to define a depth of field. The optical information must therefore be arranged at a distance from the frontal face of the reading apparatus that is typically comprised between a distance of 0 mm and a maximum work distance, defining a range that corresponds to the depth of field of the apparatus.
The illuminating means comprises one or more electronically controllable light emitters, for example LEDs, “light emitting diodes”, which are typically arranged to illuminate appropriately the optical information in the object and emit a luminous pattern of suitable power. In particular, the illuminating means has to illuminate at least the entire acquisition field of view of the reading apparatus with which it is associated, in all the corresponding depth of field.
The aiming means usually comprises a first, and optionally, a second luminous emitter, typically a laser, for emitting a luminous aiming pattern that is substantially aligned or slightly misaligned with respect to a line passing through the centre of the acquisition means, or optical axis, and having one or more luminous portions arranged with respect to this axis in such a manner as to indicate to the user, at the various reading distances, the optimum zone in which to position the optical information. In particular, the pattern can preferably indicate the centre of the field of view or an area (figure) containing the field of view or more preferably an imaginary segment the centre of which coincides with the centre of the field of view. The field of view can also be extended in relation to this area (figure) or imaginary segment.
The reading outcome indicating means is on the other hand suitable for emitting a luminous pattern, which is typically circular or has an easily recognisable shape and is coloured to provide an indication, typically green in colour, to indicate a positive reading outcome, i.e. that the image has been acquired successfully and/or that the optical information present therein has been successfully decoded.
FIG. 1 shows optical information 1 in an object (which is not shown), which in the specific case represented is a barcode. This optical information lies on a plane and is, in use, illuminated by an illuminating pattern 2 that identifies a rectangular luminous background zone of the object.
A luminous aiming pattern 3 consists of portions 3a and 3b, which are triangular in shape, the opposite vertices of which are aligned to indicate the imaginary segment the centre of which coincides with the centre of the field of view. Preferably, each triangle is equilateral.
The luminous aiming pattern is emitted at the same time as or immediately before the illuminating pattern and is in the same zone of the illuminating pattern but is of a different colour from the latter to be distinguished therefrom. The user is thus guided in positioning the reading apparatus with respect to the optical information, as the optical information is correctly framed by the image acquisition apparatus if the centre thereof corresponds substantially to the centre of the imaginary segment indicated by the aiming means.
With 4 a luminous circular reading outcome pattern is indicated, which is also usually positioned in the rectangular background zone 2, which is conventionally of a green colour and is emitted at the same time as the illuminating pattern, or immediately thereafter, but anyway alternatively to the aiming pattern 3. The reading outcome pattern 4 is in fact emitted after the aiming pattern 3 is switched off and processing of the coded information has taken place.
From what has been said above, it is clear that the visual interface HMI of the reading apparatus comprises three distinct luminous patterns distinguished by different colours, with distinct functions. As the user, whilst using the reading apparatus, also has to gaze at the visual interface for a long time, it is necessary for each luminous pattern to be easily distinguishable from the others and not to cause visual fatigue to the user.
It is observed that the aiming means comprising a laser emitter is able to emit an aiming pattern 3 bounded by well-defined contours over the entire work range of the reading apparatus, as the light emitted by the laser emitter is of the coherent type and, if appropriately collimated, enables both brilliancy and focus to be maintained over the entire work range and thus over the entire depth of field. The aiming pattern 3 is thus easily visible to a user.
Nevertheless, laser emitters are increasingly frequently being replaced by LED emitters, to reduce the costs of the reading apparatuses and to reduce eye damage. In fact, such laser emitters could, even if suitably dimensioned in such a manner as not to induce harm to the retina, temporarily compromise the sight of the person affected, if a user were to inadvertently look not at the aiming pattern 3 projected onto the plane containing the code but at the light emitted directly by the laser emitter.
If the aiming device were to comprise a LED emitter, or anyway an emitter that emits non-coherent light, the emitted luminous pattern would have a luminous intensity that diminishes as it moves away from the emission face of the reading apparatus, reducing the visibility thereof. Further, with this type of source, the optimum focus of the aiming pattern would be obtained only at a set distance and would also deteriorate moving away from this distance. The speed at which the focus deteriorates is influenced by the parameters of the optical system such as, for example, numerical aperture.
If it is desired to use a visual HMI interface comprising luminous patterns emitted exclusively by LED emitters or which are not coherent, it is thus necessary to try to improve the perception thereof by a user, at least from a distance from the frontal face of the apparatus of 50 mm, assuming that for distances comprised between 0 and 50 mm from the frontal face, the user instinctively aims the optical code at the reading apparatus, without the need to use the HMI visual interface. A significant range in which the perception of a luminous pattern has to be improved can thus be defined in the work range, from 50 mm to the maximum work distance.
In a reading apparatus of coded information comprising image acquisition means and luminous pattern emission means there are nevertheless numerous project aspects on which it is possible to intervene that influence the perception of each pattern by the human eye, above all taking into consideration the physiological features thereof.
Numerous studies exist that study the physiological features of the human eye and the response thereof to, for example, perceived luminous intensity, as in [1], or to colour as in [2], and which provide information about the luminous intensity or the ideal colour that are easily perceptible, consequently reducing the visual fatigue to the user.
Nevertheless, despite the aforesaid studies and the ideal parameters indicated therein that take account of the physiological features of the human eye, it is not possible to design the reading apparatus accordingly and, for example, set as a design specification for the reading apparatus an emitted intensity of the luminous aiming pattern that is equal to the ideal intensity. The reason lies in the fact that the perception of the aiming pattern, or of the reading outcome pattern, is also influenced by the perception of the illuminating pattern, emitted simultaneously to, or in certain applications, emitted in a close temporal relationship to the aiming pattern or to the reading outcome pattern.
Further, the perception of colour and of luminous intensity influence one another and vary, with the same colour and luminous intensity emitted, with the variation of the work distance at which the coded information to be acquired is positioned.
It should be added to what has been said above that a reading apparatus consists of complex optical and electronic components, in which all the design aspects linked, for example to the optics (focal point and numerical opening of the optical system, distance between aperture diaphragm and lens) of a projection device of the aiming pattern and of the reading outcome pattern influence one another and contribute to defining the method with which the emitted luminous patterns are perceived.
In other words, although the luminous intensity emitted by the illuminating device and by the aiming device increase, the illuminating pattern and the resulting aiming pattern might not be easily distinguishable if the corresponding emission colours are not appropriately selected, are not suitably different chromatically and if the coded information to be read is at a non-ideal work distance.